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The Hesperian is a geologic system and time period on the planet Mars characterized by widespread volcanic activity and catastrophic flooding that carved immense outflow channels across the surface. The Hesperian is an intermediate and transitional period of Martian history. During the Hesperian, Mars changed from the wetter and perhaps warmer world of the Noachian to the dry, cold, and dusty planet seen today.〔Hartmann, 2003, pp. 33-34.〕 The absolute age of the Hesperian Period is uncertain. The beginning of the period followed the end of the late heavy bombardment〔Carr, M.H.; Head, J.W. (2010). Geologic History of Mars. Earth ''Planet. Sci. Lett.,'' 294, 185–203.〕 and probably corresponds to the start of the lunar Late Imbrian period,〔Tanaka, K.L. (1986). The Stratigraphy of Mars. ''J. Geophys. Res.,'' Seventeenth Lunar and Planetary Science Conference Part 1, 91(B13), E139–E158, . http://adsabs.harvard.edu/full/1986LPSC...17..139T.〕〔Hartmann, W.K.; Neukum, G. (2001). Cratering Chronology and Evolution of Mars. In Chronology and Evolution of Mars, Kallenbach, R. et al. Eds., ''Space Science Reviews,'' 96: 105–164.〕 around 3700 million years ago (Mya). The end of the Hesperian Period is much more uncertain and could range anywhere from 3200 to 2000 Mya,〔Hartmann, W.K. (2005). Martian Cratering 8: Isochron Refinement and the Chronology of Mars. ''Icarus,'' 174, 294–320.〕 with 3000 Mya being frequently cited. The Hesperian Period is roughly coincident with the Earth’s early Archean Eon.〔 With the decline of heavy impacts at the end of the Noachian, volcanism became the primary geologic process on Mars, producing vast plains of flood basalts and broad volcanic constructs (highland paterae).〔Greeley, R.; Spudis, P., 1981. Volcanism on Mars. ''Rev. Geophys.'' 19, pp. 13–41.〕 By Hesperian times, all of the large shield volcanoes on Mars, including Olympus Mons, had begun to form.〔Werner, S.C. (2009). The Global Martian Volcanic Evolutionary History. ''Icarus,'' 201, 44–68.〕 Volcanic outgassing released large amounts of sulfur dioxide (SO2) and hydrogen sulfide (H2S) into the atmosphere, causing a transition in the style of weathering from dominantly phyllosilicate (clay) to sulfate mineralogy.〔Bibring, J.-P. ''et al.'' (2006). Global Mineralogical and Aqueous Mars History Derived from OMEGA/Mars Express Data. ''Science,'' 312(400), .〕 Liquid water became more localized in extent and turned more acidic as it interacted with SO2 and H2S to form sulfuric acid.〔Head, J.W.; Wilson, L. (2011). The Noachian-Hesperian Transition on Mars: Geological Evidence for a Punctuated Phase of Global Volcanism as a Key Driver in Climate and Atmospheric Evolution. 42nd Lunar and Planetary Science Conference (2011), Abstract #1214. http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1214.pdf.〕〔Barlow, N.G. (2010). What We Know about Mars from Its Impact Craters. ''Geol. Soc. Am. Bull.,'' 122(5/6), 644–657.〕 By the beginning of the Late Hesperian the atmosphere had probably thinned to its present density.〔 As the planet cooled, groundwater stored in the upper crust (megaregolith) began to freeze, forming a thick cryosphere overlying a deeper zone of liquid water.〔Clifford, S.M. (1993). A Model for the Hydrologic and Climatic Behavior of Water on Mars. ''J. Geophys. Res.,'' 98(E6), 10,973–11,016.〕 Subsequent volcanic or tectonic activity occasionally fractured the cryosphere, releasing enormous quantities of deep groundwater to the surface and carving huge outflow channels. Much of this water flowed into the northern hemisphere where it probably pooled to form large transient lakes or an ice covered ocean. ==Description and name origin== The ''Hesperian'' System and Period is named after Hesperia Planum, a moderately cratered highland region northeast of the Hellas basin. The type area of the Hesperian System is in the Mare Tyrrhenum quadrangle (MC-22) around . The region consists of rolling, wind-streaked plains with abundant wrinkle ridges resembling those on the lunar maria. These "ridged plains" are interpreted to be basaltic lava flows (flood basalts) that erupted from fissures.〔 The number-density of large impact craters is moderate, with about 125-200 craters greater than 5 km in diameter per million km2.〔Strom, R.G.; Croft, S.K.; Barlow, N.G. (1992) The Martian Impact Cratering Record in ''Mars,'' H.H. Kieffer ''et al.,'' Eds.; University of Arizona Press: Tucson, AZ, pp. 383–423.〕〔Tanaka, K.L. (1986). The Stratigraphy of Mars. ''J. Geophys. Res.,'' Seventeenth Lunar and Planetary Science Conference Part 1, 91(B13), E139–E158.〕 Hesperian-aged ridged plains cover roughly 30% of the Martian surface;〔 they are most prominent in Hesperia Planum, Syrtis Major Planum, Lunae Planum, Malea Planum, and the Syria-Solis-Sinai Plana in southernTharsis.〔Scott, D.H.; Tanaka, K.L. (1986). Geologic Map of the Western Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–A.〕〔Greeley, R.; Guest, J.E. (1987). Geologic Map of the Eastern Equatorial Region of Mars. U.S. Geological Survey Miscellaneous Investigations Series Map I–1802–B.〕 ImageSize = width:800 height:50 PlotArea = left:15 right:15 bottom:20 top:5 AlignBars = early Period = from:-4500 till:0 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:500 start:-4500 ScaleMinor = unit:year increment:100 start:-4500 Colors= id:prenoachicol value:rgb(0.7,0.4,1) id:noachicol value:rgb(0.5,0.5,0.8) id:hespericol value:rgb(1,0.2,0.2) id:amazonicol value:rgb(1,0.5,0.2) PlotData= align:center textcolor:black fontsize:8 mark:(line,black) width:25 shift:(0,-5) text:Amazonian from:-3000 till:0 color:amazonicol text:Hesperian from:-3700 till:-3000 color:hespericol text:Noachian from:-4100 till:-3700 color:noachicol text:Pre-Noachian from:start till:-4100 color:prenoachicol 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hesperian」の詳細全文を読む スポンサード リンク
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